Update _blocked_well.py to ensure dtype is maintained when adding grid property

[emmanesbit]

This minor edit ensures the dtype of the original property array is maintained when adding property to blocked well from a grid.

There is also an update to the unit tests for this code to add a continuous property, and check the dtype of one continuous and one discrete property are correct.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (67e4268) 80.91% compared to head (8406f6d) 80.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #733   +/-   ##
=======================================
  Coverage   80.91%   80.92%           
=======================================
  Files         190      190           
  Lines       32377    32377           
=======================================
+ Hits        26199    26200    +1     
+ Misses       6178     6177    -1     

Impacted Files	Coverage Δ
resqpy/well/_blocked_well.py	82.92% <100.00%> (ø)

... and 1 file with indirect coverage changes

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